1. Field of the Invention
The present invention relates to an imaging apparatus and a method of processing an image signal.
2. Description of Related Art
FIG. 1 shows a configuration of a past imaging apparatus 10. The imaging apparatus 10 has an imaging unit 20, a signal-processing unit 30, and an output unit 40.
An image pickup device 21 in the imaging unit 20 is driven by a driving portion 22 and produces an analog image signal SVa based on an image of a subject to supply an analog-signal-processing portion 23 with the produced image signal SVa. The analog-signal-processing portion 23 performs various kinds of signal processing such as noise reduction on the image signal SVa to produce a processed image signal SVb and supplies an A/D converter 24 with the processed image signal SVb. The A/D converter 24 receives the image signal SVb and converts it to a digital image signal DVa. A color-control-processing portion 31 in the signal-processing unit 30 receives the digital image signal DVa and performs any processing such as improvement of color reproductivity on the digital image signal DVa to produce a color-controlled image signal DVb. A nonlinear-processing portion 32 receives the color-controlled image signal DVb and performs any processing such as dynamic range compression processing and gamma processing on the image signal DVb to produce a non-linear-processed image signal DVc. A detail-processing portion 33 receives the non-linear-processed image signal DVc and performs any processing such as contour compensation on the image signal DVc to produce an image signal DVd. The output unit 40 receives the image signal DVd from the signal-processing unit 30 and converts it to a signal having a transmission format corresponding to image-displaying device and/or image-recording device to produce an output signal DVout.
FIG. 2 shows a relationship between intensity of light input to the image pickup device 21 and the imaged signal SVa output from the image pickup device 21. The image pickup device 21 used in the image pickup unit 20, which has a dynamic range of 600% or more, has been put to practical use. For example, at the intensity of input light of 0%, the image signal SVa has a signal level of LE (0%) and at the intensity of light of 600%, the image signal SVa has a signal level of LE (600%). The A/D converter 24 is thus designed for maintaining a dynamic range of 600% or more corresponding to the image pickup device 21. The dynamic range compression processing is then performed so that the image signal can have a signal level corresponding to any broadband standards.
FIG. 3 shows Knee compensation processing by which an image portion having intensity of input from 80% to 600% is compressed to an image portion having output level from 80% to 109% (white clip level). In the imaging apparatus 10, any gamma compensation is performed so that an image signal can correspond to input/output properties of display device used in the image-displaying device, not shown. For example, if a cathode ray tube is used as the image-displaying device, nonlinear processing such that its input/output property can become convex upwardly, namely, be over compensated on a region of the intensity of input of 80% or less. On a dark image region, for example, a region GA shown in FIG. 3 in which the intensity of input stays within a range from 0% to 20%, black gamma compensation is formed so that a hue in the dark image region can be richly represented or gradation in the dark image region can be finely represented. Such the Knee compensation or the gamma compensation exerts a great influence on its picture quality subjectively, thereby enabling them to be controlled according to user's desire.
Japanese Patents Nos. 2884384 and 2067650 and Japanese Patent Application Publication No. H04-319871 have respectively disclosed that the nonlinear processing such as the Knee compensation or the gamma compensation can provide an image having any subjective fine picture quality.